1. Field of the Invention
The invention relates to a blazed diffractive optical element having a support and a plurality of blazed diffraction structures which are applied on the support and are spaced apart with locally varying grating constant. The invention also relates to a projection objective for a microlithographic projection exposure apparatus comprising such an optical element.
2. Description of Related Art
Diffractive optical elements with locally varying grating constants have found many applications in optics. Diffractive optical elements are used, for example, to generate wavefront profiles which cannot—or only with great difficulty—be achieved by refractive optical elements such as lenses. Fresnel lenses which make it possible to achieve extremely short focal lengths are also widely used. The use of diffractive optical elements to correct chromatic aberrations in optical systems, which are caused by the dispersive properties of the conventional lens materials with broadband light sources, has been proposed, for example, in EP 0 965 864 A2. It is also feasible to use diffractive optical elements for focusing, collimation and beam splitting of laser light in integrated optics, since diffractive optical elements can likewise be produced photolithographically.
The grating constant g in such diffractive optical elements, by which the diffraction structures are separated from one another, may locally vary continuously or sectionally in one or more directions. The grating constant g generally varies along the diameter in rotationally symmetric elements, and along the transverse and/or long sides in rectangular elements.
One important property of diffractive optical elements is the diffraction efficiency. This term denotes the fraction of the light incident on the diffractive optical element which contributes to a particular diffraction order. For many applications, it is desirable to have a diffraction efficiency which is constant over the entire surface of the diffractive optical element. Microlithographic projection exposure apparatus, in which diffractive optical elements must ensure a very homogeneous illumination of an illuminated field, are an example of this.
It has been found, however, that the diffraction efficiency of such diffractive optical elements decreases as the grating constants become smaller. This effect is further exacerbated by the fact that the regions with the smallest grating constants are often those which the light strikes at the largest angle of incidence, and this likewise has an unfavorable effect on the diffraction efficiency. Since the grating constant in the diffractive optical elements considered here is position-dependent, the diffraction efficiency also varies over the surface of the diffractive optical element. With grating constants which are very large compared to the operating wavelength of the element, the local changes in the diffraction efficiency are small. Yet when the element also comprises regions with diffraction constants which amount only to about five times the wavelength or less, this can lead to variations of up to 20% in the diffraction efficiency.
Various measures have been proposed in order to suppress these variations.
For example, it is known from US 2002/0196547, which corresponds to DE 101 23 230 A1, to reduce the high diffraction efficiency in regions with large grating constants and thereby adapt it to the lower diffraction efficiency of the regions with small grating constants. The reduction of the diffraction efficiency is achieved by a local reduction of the grating depth. A disadvantage with this, however, is that the total usable light energy transmitted or reflected by the element is thereby also reduced. This is disadvantageous, for example, in projection exposure apparatus which have a weak radiation source and/or which have a large image field to be illuminated.
On the other hand, in certain cases it may also be expedient to use diffractive optical elements with particularly large and deliberately adjustable variations in the diffraction efficiency. If the aforementioned known approach were to be applied in such a case, then the diffraction efficiency of the regions with an already low diffraction efficiency would be reduced further in order to amplify the variations.